1. Technical Field
The present invention relates to a method of measuring the chromatic dispersion of an optical beam waveguide (hereinafter referred to as “optical waveguide”) using interference fringes of an optical interferometer, more specifically to a method of measuring the chromatic dispersion coefficient of an optical waveguide using interference fringes of an optical interferometer which allows a simple and precise measurement of a chromatic dispersion property of an optical waveguide having a short length or a small chromatic dispersion value without using a complicated measuring equipment.
2. Description of the Related Art
The chromatic dispersion of an optical waveguide is one of major parameters that influence the generation efficiency of a non-linear optical effect as well as the high-speed transmission performance of optical signals. Accordingly, it is important to precisely measure the chromatic dispersion.
There are several prior arts to measure the chromatic dispersion of an optical waveguide, which are a time of flight method, a phase shift measuring method, and an interferometer-type method.
Firstly, the “Time-of-Flight Method” measures wavelength dependent time delays of optical pulses when the optical pulses of various wavelengths are sent through an optical fiber. This method, however, is sensitive to the time jitter and the wavelength stability of the laser, and requires a high stability even in a complicated experimental structure.
Secondly, the phase shift measuring method measures a wavelength dependent phase delays after phase-modulated optical signals are allowed to pass through a long optical fiber. However, this method has a difficulty in measuring an accurate phase delay when the measurement wavelength is apart from a reference wavelength and the phase delay is large. In addition, this method has another disadvantage related to a complicated measurement setup requirement.
On the other hand, the interferometer-type measuring method measures chromatic dispersions by using an interferometer which allows a precise measurement of wavelengths, refractive indexes, lengths, and detailed spectral patterns of a light wave based on its optical interference phenomenon.
One of prior arts related to the interferometer-type measuring method [L. G. Cohen, J. Lightwave Technol., 3, 958-966 (1985)] uses a scheme of Mach-Zehnder interferometer. This method allows measurement of an absolute value of chromatic dispersion of an optical sample waveguide or fiber placed on a sample arm with an optical fiber or a waveguide having a known chromatic dispersion value placed on a reference arm and with temporal measurement of interference patterns in a time-domain when the length of the reference arm is adjusted appropriately with respect to the length of the optical sample fiber or waveguide placed on the sample arm. This method also has a drawback of requiring a reference optical fiber or a waveguide of known chromatic dispersion value to obtain an absolute chromatic dispersion value of a sample.
In another prior art related to the interferometer-type measuring method [H.-T. Shang, Electron. Lett., 17, 603-605 (1981)], a similar shape of the Mach-Zehnder interferometer as one mentioned above in the prior art [L. G. Cohen, J. Lightwave Technol., 3, 958-966 (1985)] is applied except without using any known reference waveguide or fiber, but the chromatic dispersion of the sample is determined by measuring the center of flat interference spectrum pattern which corresponds to a wavelength of zero signal delay between the reference arm beam and the sample arm beam and which is observed when the reference arm length is adjusted, and by measuring how the wavelength of zero signal delay varies with change of the reference arm length. However, since the output of the interferometer varies very sensitively to environmental instability, it is not easy to measure the wavelength of zero signal delay for both reference and sample arm beams. This kind of sensitivity to the environmental instability causes experimental measurement errors.
Yet, another prior art related to the interferometer-type measuring method described in [J. Y. Lee and D. Y. Kim, Optics Express, 14 (24), 11608-11615 (2006)] and [J. Y. Lee and D. Y. Kim, Applied Optics, 46 (29), 7289-7296 (2007)] reports measured results of the chromatic dispersion of a 55 cm-length optical fiber by using only a sample fiber placed in a Mach-Zehnder interferometer. However, this method has a drawback of a limited accuracy in measurement of the chromatic dispersion of an optical fiber or waveguide whose length is shorter than 55 cm or whose dispersion value is very small, because this method does not count any background effect in the interference pattern and phase change measurement.
Another prior art of chromatic dispersion measurement described in the patent document [U.S. Pat. No. 6,882,410] suggests a method that evaluates the minimum and maximum chromatic dispersion values of a sample by comparing the correlation between the reference signal having a known chromatic dispersion and a signal outputted from an optical time-domain reflector (OTDR). This method, however, is inconvenient in that an OTDR must be used, and the comparison with a known chromatic dispersion must be performed, and is limited to evaluation of a relative minimum and maximum chromatic dispersion instead of obtaining the absolute chromatic dispersion.
In addition, another prior art of chromatic dispersion measurement described in the patent document [U.S. Pat. No. 6,943,871] suggests a method that evaluates the chromatic dispersion from the change of the interferometer output according to the modulation frequency of an optical modulator in a Sagnac loop interferometer. However, this method is unable to make a detail suggestion for measuring the chromatic dispersion of an optical fiber or optical waveguide having a short length.
Recently the precise measurement of the chromatic dispersion is required to maximize the nonlinear effect in a silicon optical waveguide. The length of the silicon optical waveguide is about several centimeters or smaller. However, the aforementioned documents related to prior arts on measurement of the chromatic dispersion do not provide any solution for precise measurement of chromatic dispersion of the optical waveguide having a length of several centimeters or smaller.